Assuming you have experience with other programming languages, the way that worked best for me to get up to speed with PHP:

1) Go and buy (or borrow from your local library) a book like "Learn PHP in 24 hours"

2) go on to SourceForge (sourceforge.net) and search for a PHP project that does something you want eg - a DVD library, a meeting room booking system etc. Download it and then use that as a basis to modify it to do what you want. I find reading someone else's code and modifying it to meet my needs the quickest way to learn a new programming language.